1. Field of the Invention
The present invention relates to the imaging of high temperature objects, and more specifically, it relates to the use of a Faraday filter to image high temperature objects.
2. Description of Related Art
Viewing of high temperature, self-luminous objects is difficult because the contrast is either too high (welding) or too low (blast furnace). Bandpass filters are used in optical systems in conjunction with laser light sources in order to exploit the narrow band properties of lasers. The filters selectively attenuate background ambient light, which is broadband, relative to the laser light.
The dielectric filter is commonly used in optical systems in conjunction with laser light sources because of its economy and effectiveness. However, the bandwidth of a dielectric filter is orders of magnitude wider than the line widths of most lasers, so the potential for spectral discrimination is not fully realized. Narrow band dielectric interference filters typically have bandwidths of 10 nm (FWHM) and peak transmissions of 50%. Narrower bandwidths are possible, but the filters become excessively sensitive to temperature and field of view, and peak transmission is reduced substantially. Integrated out-of-band transmission, or leakage, is suppressed by the use of absorbing glass cutoff filters packaged with the dielectric filter, and is generally several orders of magnitude less than integrated inband transmission.
Techniques and products using laser illumination and filters exist and are commercially sold. For example Control Vision Incorporated (841 Chamberlain Ave. Idaho Falls, Ida.) has a line of products using high peak power lasers as illuminators. However these systems use conventional narrow band filters which have a wide bandpass of typically 5 nm and need more expensive high power illuminators to overcome the background radiation. The much wider bandpass requires that the laser be pulsed and the detector gated. Both add significant cost and complication to the system. In U.S. Pat. No. 5,202,741, advantage was taken of the fact that basically only the object would reflect laser radiation and thus become detectable.
Faraday filters provide ultra-narrow optical bandwidths of the order of a few GHz with offband rejection of 10.sup.5 and throughput of 70%. They utilize the anomalous dispersion of an atomic absorption line in the presence of an axial magnetic field, and are therefore available at numerous discrete wavelengths throughout the optical spectrum. As shown in FIG. 1, a Faraday filter 2 using the cesium resonant line at 852 nm would consist of a pair of crossed polarizers 4 and 6 on either side of a vapor cell 8 heated to about 120.degree. C. The vapor cell is mounted inside a solenoid 9 providing an axial magnetic field of around 100 gauss.